1. Field of the Invention
This invention relates to a process for upgrading heavy hydrocarbonaceous oils, and more particularly to an improved hydrogen donor diluent cracking process.
2. Description of the Prior Art
The hydrogen donor diluent cracking process in which certain low value hydrocarbon fractions are upgraded by thermal cracking in the presence of a hydrogen donor diluent is described in detail in U.S. Pat. No. 2,953,513. Process variables and operating conditions for the hydrogen donor diluent cracking process are discussed at length in that patent. One disadvantage of the conventional hydrogen donor diluent cracking process is that fractionation of the product stream from the cracking step produces a heavy bottoms or pitch stream which is of low value relative to the other products from the process and in some cases presents a disposal problem. A portion of this heavy pitch material may be recycled through the process, but as a practical matter a substantial purge is usually required in order to operate the process continuously in an efficient manner. The process of the present invention overcomes the aforementioned disadvantage, and provides a more efficient process.
In U.S. Pat. No. 4,115,246 a process is described in which the pitch fraction resulting from fractional distillation of the products of a hydrogen donor diluent cracking step is subjected to a partial oxidation process, and the resulting hydrogen-containing gas produced by the partial oxidation step is utilized to hydrogenate the recycled hydrogen donor solvent. The pitch fraction is defined as the product of the fractional distillation boiling above 500.degree. C. The fresh feedstock to the cracking furnace could be shale oil, tar sand bitumen, or residual oil from a petroleum refining operation.
U.S. Pat. Nos. 4,294,686 and 4,363,716 each disclose hydrogen donor diluent cracking processes in which part of the pitch fraction from the cracking step is recycled back to the cracking zone.
In U.S. patent application Ser. No. 365,721 filed Apr. 5, 1982 and assigned to the assignee of the present application, a hydrogen donor diluent cracking process is described in which the pitch fraction from the cracking step is heat-soaked with donor solvent and then returned to the cracking zone.